Subterranean termites most often enter structures from the surrounding soil to feed on wood, or other cellulosic material, of the structure and its contents. If unchecked, termites can cause considerable damage. As a result, efforts to erect physical or chemical barriers to prevent the entrance of termites into a structure or to exterminate the termites after they have invaded a structure have proven a considerable expense to the public.
Subterranean termites construct an extensive foraging gallery beneath the soil surface. A single colony may contain several million termites with foraging territory extending hundreds of feet (Su, N.Y., R. H. Scheffrahn [1988] Sociobiol. 14(2):353-359). Because subterranean termites are cryptic creatures, their presence is not normally known until after some damage, foraging tubes, or live termites (such as swarmers) are found.
Currently, there are two basic approaches for the control of subterranean termites: preventive control and remedial control. In some of the United States, it is mandatory that the soil underlying the foundation of newly constructed buildings be pre-treated with a pesticide (also referred to herein as termiticide) to prevent termite infestation. However, the barrier often loses its continuity, and the currently available soil termiticides tend to lose their biological activity after a few years. Established termite colonies in the soil may then invade the structure if additional chemical is not applied beneath and around the structure.
When a house or other building is infested by subterranean termites, one option is to introduce termiticide around a building foundation by injection into soil underlying concrete foundations, drenching the soil surrounding the building perimeter, or a combination of both. This type of post-construction treatment is labor-intensive and may not adequately produce a continuous barrier. Other remedial treatments include spot treatments such as dusting or injecting termiticides (such s arsenic trioxide) within the walls of the building.
Some toxicants that have less environmental effect and that show activity against termites are known. However, prior to 1995, these toxicants were not used in conjunction with a method that efficiently and efficaciously delivered the toxicant to a target pest.
The introduction of the first commercial termite bait system, SENTRICON®, in 1995 drastically changed subterranean termite control practices. Unlike prior treatments, a monitoring-baiting program such as SENTRICON® is capable of eliminating the entire colony. See WO 93/23998, U.S. Pat. No. 6,370,812, and U.S. Pat. No. 6,397,516. Because of the target-specific approach, only a few grams of hexaflumuron are needed to eliminate a colony that may contain several hundred thousand termites (Su 1994, J. Econ. Entomol. 87:389-397). Due to its low environmental impact, hexaflumuron was the first compound to be registered under EPA's Reduced Risk Pesticide Initiative. The SENTRICON® system was the recipient of EPA's Presidential Green Chemistry Award in 2000.
There remains a need for additional termite toxicants that have little or no adverse environmental effects. For example, U.S. Pat. Nos. 5,753,249; 6,214,364; and 5,558,862 relate to methods of controlling insects by administering enzymes that disrupt ecdysteroid metabolic pathways.
Insects have an exoskeleton (made of mostly chitin and proteins) that protects them from external elements such as weather and natural enemies. The external cuticle, however, has to be shed periodically for continuous growth. Endocrine products, most notably brain hormone, juvenile hormone, and ecdysone (FIG. 1A), are known to regulate insect molting (Chapman 1976). Ecdysone is rapidly converted to 20-hydroxyecdysone (or “20E,” FIG. 1B) following its release into the insect hemolymph (Nation 2002). The presence of juvenile hormone (JH), a sesquiterpenoid, ensures that insects retain the juvenile form (i.e. to molt from a younger larval stage to the next larval stage). As insects progressively molt, JH concentration decreases and may even be totally absent at the last larval instar. In the absence of JH, a larva then molts into pupa or adult stage.
The effects of JH, its analogs (JHAs), and mimicries (JHMs) on termites are well studied (Su and Scheffrahn 1990). JHAs and JHMs (referred to as juvenoids) are known to produce excessive soldier termites whose function is for colony defense. Because the soldier caste has to be fed by workers, termite colonies contain optimal proportions of the soldier caste (Wilson 1971, Haverty 1977). It has been proposed that juvenoids, which induce excessive soldier formation, may be used to disrupt the integrity of a termite society, leading to the destruction of the entire colony (Haverty 1977, Hrdy and Krecek 1972, Hrdy 1973). However, further studies revealed that juvenoids are effective only against termite species with a lower natural soldier proportion, such as Reticulitermes species) (Su and Scheffrahn 1990). Coptotermes species, which have a relatively high proportion of soldiers, include an unusually large proportion of economically important termites in the world (Su 2003).
One relatively recent development in subterranean termite control is the use of termite baits containing chitin synthesis inhibitors (CSIs) such as hexaflumuron or noviflumuron to eliminate the vast colony of subterranean termites (Su 1994, Su 2003). In order to eliminate an entire colony of termites, the active ingredient (AI) for a bait has to be non-repellent, slow-acting, and its lethal time has to be dose-independent so that the AI is distributed throughout the colony by termites before the onset of death (Su and Scheffrahn 1998). Insect growth regulators (IGRs) such as juvenoids and CSIs satisfy all these three requirements. However, many IGRs are species-specific (Su 2003). CSIs generally inhibit the biosynthesis of chitin, but its complete process remains poorly understood (Nation 2002).
U.S. Pat. No. 6,093,415 relates to synergistic effects between juvenoid insecticides and CSIs in termite baits. Ecdysone and analogs thereof are not mentioned.
While juvenoids and CSIs are well investigated for their potentials for termite control, limited information is available for even the normal function of ecdysone in termites. Lüscher and Karson (1958) and Lüscher (1960), while trying to determine the role, if any, that ecdysone plays in the normal biology of termites (and not in attempts to control termites), reported that injection of ecdysone alone or in combination with JH induced a normal pseudogate molting of the lower termites, Kalotermes flavicollis. Since their studies, there has been no known investigation of the effects of ecdysone in or on termites.
Synthetic versions of ecdysteroids have been used to control some insect pests of agricultural importance, but not termites. These analogs typically mimic the activity of 20E to cause premature molting (Wing et al. 1988). Toward the end of the molting under normal condition, 20E is degraded and excreted, thus allowing the eclosion hormone to complete the process (Nation 2002). The analogs, however, are more stable than 20E and are not easily degraded or excreted (Wing et al. 1998). Consequently, their continuing presence in insect hemolymph interferes with complete molting and causes hyperecdysonism (premature molting without a successful termination). Hence, these analogs can be referred to as ecdysteroid agonists (Dhadialla et al. 1998).
U.S. Pat. Nos. 6,123,756 and 6,248,159 relate to wood preservative for protecting wood against dry-wood-destroying insects, such as the house longhom (Hylotrupes bajulus), woodworm (Anobium punctatum), and bark beetle (Lyctus brunneus). Those patents relate to lumber (not insect bait) treated with a combination of a juvenile hormone and an ecdysone agonist. Subterranean termites are different from drywood termites, are not dry-wood-destroying insects, and are not mentioned or suggested in those patents. See, e.g., U.S. Pat. No. 5,027,546, which describes a system intended for use on above ground termites, i.e. drywood termites, by freezing them with liquid nitrogen.
Positive results were obtained when the ecdysteroid agonist RH-5849 was tested against some insect species (Darvas et al. 1992), but limited information is available regarding potential effects of ecdysteroid agonists against termites. Raina et al. (2003) reported that one such agonist, halofenozide, may impact the reproductive physiology of alate nymphs of C. formosanus. However, for use in baits to eliminate a subterranean termite colony, the active ingredient in the bait has to be lethal to the worker caste, which makes up the majority of the colony population. Alate nymphs, which make up only a small portion of a colony, eventually leave the nest to start a new colony somewhere else. However, they do not forage like workers. Thus, eliminating the young, alate nymphs would not impact the overall colony population and its damaging potential.
Halofenozide is one of several molt-accelerating compounds (MACs) owned by Dow AgroSciences. Halofenozide has activity on Lepidoptera and some Coleoptera species. In the urban pest management business, halofenozide is currently registered under the trade name MACH 2™ in the United States for control of white grub and surface feeding Lepidoptera in residential and commercial lawns and golf courses.
There have been some limited reports of testing of halofenozide against termites. For example, an abstract from the Florida Entomological Society 2002 Annual Meeting (Monteagudo & Su [2002]), related to preliminary results of a choice test conducted to examine preference, deterrence, and lethality of the insect growth regulator bait halofenozide on workers of the eastern subterranean termites (Reticulitermes flavipes). The corresponding “ten-minute paper” stated that halofenozide was evaluated in a choice test for its potential as a bait toxicant against the eastern subterranean termite. Feeding blocks composed of wood slices were vacuum-impregnated with halofenozide at various concentrations, with feeding deterrence occurring at concentrations greater than 4,000 ppm. There was also a USDA inhouse memo that mentioned RH-0345 (a.k.a. halofenozide) as having a significant effect on ovarian development in alates and nymphs of C. formosanus. However, the effect, at low doses used in the test, was temporary. That memo also discussed the use of a juvenile hormone analog to induce the formation of mutant soldiers. The USDA termite program in New Orleans has also examined halofenozide for potential ovicidal effects in C. formosanus and presented results in 10-minute oral presentation at the 2001 meeting of the Entomological Society of America (paper 0269).
There have been no known reports regarding testing ecdysteroids or analogs thereof, other than halofenozide, for termite control. Furthermore, there are no known tests regarding the combination of the CSI+MAC (or ecdysteroids) classes of chemistry against insects, including termites.